A fundamental problem in cell biology is to understand how cells modulate both temporally and quantitatively the production of proteins required for maintaining the organization of the cytoplasm and for providing motile force. Using as a model the regeneration of the paired anterior flagella of the unicellular alga Chlamydomonas, this project addresses one segment of this control pathway, namely the regulation of messenger RNA abundance. Flagellar mRNAs are present at low levels in nondeflagellated cells and accumulate rapidly after deflagellation, peaking in about one hour at a tenfold higher level. An hour after that, they have been degraded and predeflagellation levels of these RNAs are reestablished. The mechanism by which transcription of flagellar protein genes is coordinated will be investigated using a combination of comparative DNA sequence analysis and replacement of recombinant gene segments into the cells. These studies will focus on DNA sequences that are present in flagellar protein genes, but not in genes whose transcription remains unaffected by flagellar loss. In another set of experiments the messenger RNAs will be compared to determine how flagellar mRNA species, whose fate is to be rapidly degraded, differ structurally and functionally from nonflagellar mRNA species that remain relatively stable in the cytoplasm of regenerating cells. Terminal, untranslated nucleotide sequences of the RNAs, proteins bound to the RNAs, and association of the RNAs with polysomes will be studied. Based on this work, further experiments will address specific mechanisms by which mRNA degradation is controlled. The results of these studies on control of coordinate gene transcription and selective mRNA degradation are relevant not only to the goal set forth here, which is to understand cytoskeletal elaboration, but also to the general problem of control of gene expression. Molecular mechanisms similar to those responsible for flagellar mRNA abundance regulation are likely to be applicable to metabolic changes and developmental events associated with normal and disease processes.